fix(hotkey): split platform files by platform+cgo build matrix

The `!darwin` guard on the modifier helpers was too broad: golang.design/x/
hotkey only exposes its Mod*/Key* constants from the darwin&&cgo, linux&&cgo
and windows backends, while the `!windows && !cgo` complement (e.g. Linux
cross-builds with CGO_ENABLED=0) falls back to a stub with no constants. That
made hk.ModCtrl/ModShift/KeyA... undefined and produced editor/vet warnings.

Realign the build tags with the upstream matrix and confine every hk.* constant
to a tagged file so the shared hotkey.go compiles everywhere. Also fix the
Linux mapping (option -> hk.Mod1; hk.ModAlt does not exist on X11).

internal/infrastructure/hotkey/hotkey_unsupported.go

@@ -0,0 +1,25 @@
+//go:build !windows && !cgo
+
+package hotkey
+
+import hk "golang.design/x/hotkey"
+
+// This file covers the configuration where golang.design/x/hotkey compiles its
+// cgo-less stub (any non-Windows platform built with CGO_ENABLED=0). In that
+// stub the package defines the Modifier/Key types but no Mod*/Key* constants,
+// and Register panics at runtime. We still provide the package-internal
+// helpers so that snapgo keeps cross-compiling (e.g. `go vet`, editor analysis,
+// or CI cross-builds) without pulling in a C toolchain.
+//
+// The returned values are inert zero modifiers and lookupKey always reports
+// "unsupported": callers on this build cannot register a real global hotkey,
+// so surfacing an "unsupported token" parse error is preferable to reaching
+// the upstream panic.
+func modCmd() hk.Modifier    { return 0 }
+func modCtrl() hk.Modifier   { return 0 }
+func modOption() hk.Modifier { return 0 }
+func modShift() hk.Modifier  { return 0 }
+
+// lookupKey has no key constants to map against in the cgo-less stub, so it
+// always reports the token as unsupported.
+func lookupKey(string) (hk.Key, bool) { return 0, false }